[ONNX] Fix pow op export (#38065)
Summary:
Fix pow type cast for opset 9 and update opset 12
Pull Request resolved: https://github.com/pytorch/pytorch/pull/38065
Differential Revision: D21485353
Pulled By: malfet
fbshipit-source-id: 3993e835ffad07b2e6585eb5cf1cb7c8474de2ec
